Practically all discharge lamps utilize mercury enclosed within the discharge lamp vessel which vaporizes during operation. In high-pressure discharge lamps, the mercury is introduced in the form of a halide mercury compound, or by directly introducing a drop through an exhaust tube into the discharge vessel. In low-pressure discharge lamps, for example fluorescent lamps, it is customary to either use direct drop-in introduction of mercury or to use containers made of glass or metal into which the mercury or the mercury alloy is filled. The container is secured close to an electrode within the interior of the discharge vessel, and by induced high frequency or laser beams is opened after the vessel has been sealed to permit the mercury to escape. U.S. Pat. No. 4,282,455 illustrates such an arrangement and process.
It is extremely difficult, and practically impossible, to properly proportion minute amounts of mercury due to the high surface tension. Thus, dosing or measuring liquid mercury in minute quantities is very difficult. Consequently, the lamps will retain in many instances a substantially larger quantity of mercury than is actually needed for operation. Direct introduction of liquid mercury is also impeded by retention of mercury drops on the stem of an exhaust tube. This may occur when the drops are below a predetermined small minimum size.
It has been proposed by colleagues of the inventor hereof to utilize a process in which liquid mercury is formed in a long strip, for example in a trough, and cooled below its freezing point. The frozen filamentary or rod-like mercury then is divided into parts having the requisite length. In other words, the required quantity of mercury is severed from the frozen strand, and the so severed part introduced into the discharge vessel. This method provides for much better and more accurate proportioning and introduction of the appropriate amount of mercury. However, it has also been found that in high-volume production, it is very difficult to be carried out since the required cooling and cold-introduction elements must be integrated with existing lamp production machinery, which is difficult to achieve without substantial redesign of expensive large-scale high-production apparatus.
Handling liquid mercury is not only dangerous but also toxic; handling such liquid mercury thus results in placing substantial stresses on the environment and working places. Mercury has a relatively high vapor pressure, and the vapor is highly toxic. If a drop of mercury falls off a container on a hard surface, it has the tendency to spray off into tiny droplets which are extremely difficult to retrieve.